1. Field of the Invention
The present invention relates to a process and apparatus for the preparation of poly-p-phenylene terephthalamide (hereinafter referred to as "PPTA" for brevity) fibers. More particularly, the present invention relates to a high-speed spinning process for preparing PPTA fibers having improved mechanical properties at a high efficiency at an industrially advantageous speed, and to a spinning apparatus for use in such a process.
2. Description of the Prior Art
It is known that wholly aromatic polyamides are derived from an aromatic diamine and an aromatic dicarboxylic acid and/or an aromatic aminocarboxylic acid, and it also is known that fibers are obtained from these aromatic polyamides. Furthermore, it is known that of these aromatic polyamides, PPTA type polymers provide fibers having such preferred properties as high melting point, excellent crystallinity, high strength and high Young's modulus, as expected from the rigid molecular structure thereof.
For example, U.S. Pat. No. 3,767,756 teaches that fibers having preferred mechanical properties can be obtained by extruding an optically anisotropic solution of a PPTA type polymer in concentrated sulfuric acid having a concentration of at least 98% through an orifice into an inert non-coagulating fluid and then passing the extrudate through a coagulating bath. In this process, however, a large take-up tension, that is, a large spinning tension, is imposed on filaments by the frictional resistance between the coagulating solution in the coagulating bath and the travelling filaments. This spinning tension is increased with increase of the spinning speed. Accordingly, under a low spinning tension, that is, at a low spinning speed, fibers having excellent mechanical properties can be obtained, but with increase of the spinning speed, the strength and elongation of the obtained fibers are drastically reduced. Therefore, it is very difficult to obtain PPTA fibers having excellent mechanical properties at an industrially significant spinning speed.
As means for reducing the spinning tension which tends to extremely increase with increase of the spinning speed, there has been proposed a process in which a specifically designed spin tube having a fine hole is arranged in the lower portion of the coagulating bath and spinning is carried out while letting filaments and the coagulating solution simultaneously fall down (see U.S. Pat. No. 4,078,034). Even if this process is adopted, however, it is impossible to sufficiently reduce the tension and obtain fibers having high mechanical properties at a high spinning speed, especially a spinning speed higher than 300 m/min.
As means for reducing the frictional resistance caused by the speed difference between the coagulating solution and the filaments in a high spinning speed region, there have been proposed a process in which the solution in the coagulating bath is compressed to accelerate the coagulating solution extruded from the spin tube (see U.S. Pat. No. 4,070,431) and a process in which another coagulating solution jetted from a plurality of small-diameter nozzles or slits is caused to impinge in the yarn take-up direction against the filaments and accompanying coagulating solution stream falling down through the spin tube, whereby the coagulating solution stream is accelerated (see Japanese Unexamined Patent Publication (Kokai) No. 56-128312). It is possible to seemingly reduce the spinning tension by accelerating the coagulating solution. However, especially in the latter process, jets of the jetted coagulating solution are applied as excessive tensions to parts of the filaments to cause fracture of the higher order structure of the filaments in which coagulation is still incomplete, with the result that the strength and elongation are reduced and fibers having sufficiently high performances cannot be obtained.
As means for reducing the spinning tension, there has been proposed a process in which a spin tube is located at a very shallow position in the coagulating bath to decrease the amount of the coagulating solution falling down together with filaments and if necessary, a specific amount of another coagulating solution is jetted in the yarn take-up direction and caused to impinge against the filaments and coagulating solution falling down through the spin tube, whereby acceleration is effected (see Japanese Unexamined Patent Publication (Kokai) No. 57-121612). In this process, however, since the coagulating bath is shallow and the amount of the falling coagulating solution is reduced, the coagulation is more incomplete, and even if the tension is reduced, crystal orientation in the filaments and fracture of the higher order structure of the filaments are advanced simultaneously, resulting in reduction of the strength and elongation. Even if reduction of the strength is controlled by the effect of reducing the tension, only fibers having a low elongation are obtained. Of course, this tendency becomes conspicuous in a high spinning speed region because the force of inertia is increased with increase of the spinning speed. Furthermore, when an aqueous solution of sulfuric acid, use of which is very advantageous from the industrial viewpoint, is used as the coagulating solution, since the advance of coagulation is delayed at a high spinning speed, the above-mentioned tendency becomes more conspicuous and as the result, high performance PPTA fibers applicable to practical use cannot be obtained.
It is well-known that PPTA fibers applicable to practical use should have not only high strength but also a high elongation, and these two properties are especially important for the fatigue resistance when the fibers are used for tire cords.